Organic semiconducting (OSC) materials are receiving growing interest mostly due to their rapid development in the recent years and the lucrative commercial prospects of organic electronics.
One particular area of importance is organic photovoltaics (OPV). Conjugated polymers have found use in OPVs as they allow devices to be manufactured by solution-processing techniques such as spin casting, dip coating or ink jet printing. Solution processing can be carried out cheaper and on a larger scale compared to the evaporative techniques used to make inorganic thin film devices. Currently, polymer based photovoltaic devices are achieving efficiencies above 8%.
In order to obtain ideal solution-processible OSC molecules two basic features are essential, firstly a rigid π-conjugated core or backbone, and secondly suitable functionality of the aromatic cores in the OSC backbone. The former extends π-π overlaps, defines the primary energy levels of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO), enables both charge injection and transport, and facilitates optical absorption. The latter further fine-tunes the energy levers and enables solubility and hence processability of the materials as well as π-π interactions of the molecular backbones in the solid state.
A high degree of planarity reduces the energetic disorder of OSC backbones and accordingly enhances charge carrier mobilities. In prior art most of the polymeric OSCs with high charge carries mobilities are generally composed of fused ring aromatic systems, and are semicrystalline in their solid states. Such polymers are for example indacenodithiophene-benzothiadiazole copolymers, for which it was reported by Zhang et al., J. Am. Chem. Soc., 2010, 132(33), 11437 that a hole mobility of 1 cm2/V s was achieved.
Nevertheless, the structures of solubilising groups (e.g., the length, the regio-regularity, the spacial orientation of the alkyl chains etc.), have direct effects on the solubility and hence the processibility of the OSC, on the planarity of the polymer backbone, on the inter-chain π-π interactions and on the HOMO-LUMO levels/bandgaps. For many applications, like e.g. OPV devices, optimisation of the electronic properties of the conjugated backbones by fine-tuning the solubilising functional groups can result in dramatic effects on the efficiencies.
The conventional method of introducing solubilising groups into cyclopentadiarene units like indacenodithiophene (Zhang et al., J. Am. Chem. Soc., 2010, 132(33), 11437), is to alkylate the sp3 carbon atoms of the cyclopentadienes contained in these fused ring structures. Due to the tetrahedral configuration of this carbon, the substituents have to take the orientation within a plane that is normal to the aromatic plane of the conjugated backbone, as shown by X-ray single crystal analysis by Hughes et al., Org. Biomol. Chem., 2003, 1, 3069. These out-of-plane alkyl chains increase the inter-planar separation of the π-π backbones, reducing the degree of inter-molecular π-π interactions. However, from a synthetic point of view, multiple alkylation like for example tetraalkylation of the indacenodithiophene leads to difficulties of purication of the expected products due to the very similar polarities of the product and the incompletely alkylated impurities.
Thus there is still a need for organic semiconducting (OSC) materials that are easy to synthesize, especially by methods suitable for mass production, show good structural organization and film-forming properties, exhibit good electronic properties, especially a high charge carrier mobility, good processibility, especially a high solubility in organic solvents, and high stability in air. Especially for use in OPV cells, there is a need for OSC materials having a low bandgap, which enable improved light harvesting by the photoactive layer and can lead to higher cell efficiencies, compared to the polymers from prior art.
It was an aim of the present invention to provide compounds for use as organic semiconducting materials that do not have the drawbacks of prior art materials as described above, are easy to synthesize, especially by methods suitable for mass production, and do especially show good processibility, high stability, good solubility in organic solvents, high charge carrier mobility, and a low bandgap. Another aim of the invention was to extend the pool of OSC materials available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
The inventors of the present invention have found that one or more of the above aims can be achieved by providing compounds, including small molecules, oligomers and conjugated polymers, containing one or more 4,9-dialkylidene-s-indacenodiheteroarene-2,7-diyl repeating units having the following structure
wherein R is e.g. alkyl or fluoroalkyl groups and X is e.g. S or Se.
The dialkylidene-s-indacenodiheteroarene as claimed in the present invention are solubilised using alkylidene groups R, of which the carbon atoms connecting to the ring systems are sp2-hybridized instead of sp3-hybridized. The sp2-carbons permit the solubilising alkyl chains to adopt a coplanar conformation relative to the core/polymer backbone, thus facilitating cofacial aggregation in the solid state. This kind of coplanar orientation of the alkyls has been demonstrated by the crystal structures of polymers as disclosed in the examples of the present invention.
It was found that compounds comprising dialkylidene-s-indacenodiheteroarene units are attractive candidates for photovoltaic applications, specifically in bulk heterojunction (BHJ) photovoltaic devices. By the incorporation of the electron-donating dialkylidene-s-indacenodiheteroarene unit and an electron-accepting unit into a co-polymer i.e. a “donor-acceptor” polymer, a reduction of the bandgap can be achieved, which enables improved light harvesting properties in bulk heterojunction (BHJ) photovoltaic devices. Also, by varying the substituents at the alkylidene group the solubility and electronic properties of the compounds can be further optimised.
In prior art it has so far not been suggested using alkylidene and arylidene groups to solubilise indacenodiarene units. Usta et al., J. Am. Chem. Soc., 2008, 130(7), 8580; Tian et al., J. Mater. Chem., 2010, 20(37), 7998 and CN 101798310 A disclose monomeric dicyano substituted derivatives, and WO 2011/025454 A1 discloses polymeric dialkoxycarbonyl substituted derivatives, of indacenodithiophene or indacenofluorene.
However, there are significant differences between the compounds of the present invention and the compounds disclosed in prior art. Firstly, the alkylidene groups in the compounds of the present invention function as solubilising groups and the corresponding substituted indacenodiarenes remain π-electron donating units, whereas in the compounds of prior art, the electron-withdrawing cyano or alkoxycarbonyl substituents yield indacenodiarenes that behave as π-electron accepting units. Secondly, the current invention provides synthesis methods using aldehydes and ketones to incorporate the alkylidenes groups onto the target substrates under alkaline conditions, whereas the prior art discloses methods using the quinoid forms of indacenodiarenes reacting with either malononitrile or malonic esters under acidic conditions, which generally give poor yields with limited choice of structural variations.